1. Field of the Invention
WO2006/075167 and WO2009/122184 (both the Secretary of State for Defence and both incorporated herein by reference) describe, inter-alia, acoustic reflectors which can be used in a variety of ways to mark underwater structures, objects or geological features.
2. Discussion of Prior Art
These applications describe an acoustic reflector comprising a shell surrounding a core, said shell being capable of transmitting acoustic waves incident on the surface of the shell into the core to be focused and reflected from an area of the shell located opposite to the area of incidence so as to provide a reflected acoustic signal output from the reflector, having a core in which the shell is dimensioned relative to the core such that a portion of the acoustic waves incident on the shell wall are coupled into the shell and guided therein around the circumference of the shell and then re-radiated to combine constructively with the said reflected acoustic signal output so as to provide an enhanced reflected acoustic signal output in which the acoustic wave speed in the core is between 840 metres per second and 1500 metres per second. However, such reflectors are normally omni-directional, and can provide little information about the specific reflector concerned, its environment or the relative position of the source of acoustic waves with respect to the reflector.